NORTHAMPTON -- Sen. Elizabeth Warren's endorsement of Hillary Clinton has sparked more backlash in Northampton -- this time in the form of roadside graffiti.

On Sunday, a small crowd of Bernie Sanders supporters dressed in black and demonstrated silently as Warren delivered a speech at the World War II club, in outrage over Warren's support for Clinton.

And now, graffiti reading "#JudasWarren Sellout" in large red letters marks a concrete structure on Route 5 just before the entrance to I-91.

Warren, long hailed by the Democratic party's left wing as one of the caucus' strongest progressive voices, endorsed Clinton following her victory in the California primary -- a result that granted Clinton a majority of the pledged delegates in the race and established her as the party's presumptive nominee.

But while the Massachusetts Senator has proven willing to jump into the campaign fray, launching public broadsides on Twitter against presumptive Republican nominee Donald Trump, she had largely stayed quiet on the Democratic race prior to her endorsement of Clinton.

Jamie Guerin, a Sanders supporter and organizer of Sunday's demonstration, said that while she supported Warren's positions she felt betrayed by her endorsement of Clinton.

"A lot of us are very disappointed," Guerin said on Sunday. "We were kind of shocked that she didn't step up and endorse Bernie Sanders sooner before the Massachusetts primary."

In the same interview, Guerin said the demonstration outside Warren's speech was not a personal attack against her. After the group met and spoke with Warren after her speech, Guerin said.

"[The graffiti is] nothing that I'm a part of, or anybody in my group is a part of," Guerin added Friday. "That's nothing that we would do to get our message across the Elizabeth Warren."

Both Sanders' campaign and Warren's office did not return requests for comment prior to publication.

Northampton Police Capt. John Cartledge said he was not aware of any complaints about the graffiti, but would send an officer to take a report.

#JudasWarren, the Twitter hashtag included in the graffiti, includes Sanders supporters displaying anger and disappointment about Warren's endorsement while describing her as a sellout and a backstabber.

Sanders has left open the possibility of taking his campaign to the Democratic Convention in Philadelphia in late July, despite Clinton's pledged delegate lead and a survey showing overwhelming superdelegate support for her candidacy.

Sanders met with Clinton Tuesday night and promised to help defeat presumptive Republican nominee Donald Trump in the general election, but has not dropped out of the race or endorsed her.

Before her speech on Sunday, which focused on income inequality, Warren expressed grief over the mass shooting that morning at the Pulse gay nightclub in Orlando, which claimed the lives of 49 victims during a Latin night.

She declined to comment on the Sanders demonstrators outside, saying it was not a time for campaign chatter.